sample函数中如果不指定prob，怎么抽样？均匀抽样？help中说“The optional prob argument can be used to give a vector of weights for obtaining the elements of the vector being sampled. They need not sum to one, but they should be non-negative and not all zero.”--“They need not sum to one”，怎么个用法？运行的时候“归一化”？